It is well known that certain metals, particularly iron-based alloys, tend to become brittle when in contact with carbon at elevated temperatures. This condition is commonly referred to as carburization of the metal. A similar effect from exposure to sulfur has been noted and referred to as sulfidation.
The former is better known and recognized. However, the latter has recently been discussed by J. Barnes et al. in an article entitled "Sulfur Effects on the Internal Carburization of Fe--Ni--Cr Alloys" at pages 333-349 of Vol. 26, Nos. 5/6, 1986, Oxidation of Metals. The authors observed the formation of a surface layer of chromium sulfide which reduced internal carburization, but became a problem itself.
It is evident that carbon and sulfide can behave in similar manner in attacking certain alloys. The present invention arose from an effort to protect against carburization, and the invention is described with reference thereto. Nevertheless, it will be appreciated that protection against both carburization and sulfidation is needed, and that the invention provides protection against both.
The problem of metal protection, particularly the protection of stainless steels used in elevated-temperature environments, is common to many industrial applications. A typical example is carburization, with or without oxidation, in stainless steel, resistance-heating elements and various components and fixtures of heating furnaces. The simultaneous carburization and oxidation of stainless steel heating elements basically results from precipitation of chromium as chromium carbide. This is followed by oxidation of the carbide particles, and is common in nickel-chromium and nickel-chromium-iron alloys. As reported by Barnes et al., chromium sulfide also forms when sulfur is present.
Another typical industrial example involves the carburization of certain petrochemical-plant components, such as heater tubes. These tubes may be subjected to carburization, oxidation, or both. These two processes generate uneven volume changes, which result in very high internal stresses, together with metal loss. They also cause embrittlement due to carbon pickup and consequent carbide formation. Ultimately, the carbide formation results in a loss of ductility that renders the component susceptible to brittle fracture. Heavy carburization also eliminates the possibility of repair welding. The present invention is described with particular reference to this application.
At the heart of a thermal cracking process is the pyrolysis furnace. This furnace comprises a fire box through which runs a serpentine array of tubing. This array is composed of lengths of tubing and fittings that may total several hundred meters in length. The array of tubing is heated to a carefully monitored temperature by the fire box.
A stream of feedstock is forced through the heated tubing under pressure and at a high velocity, and the product quenched as it exits. For olefin production, the feedstock is frequently diluted with steam. The mixture is passed through the tubing array which is commonly operated at a temperature of at least 750.degree. C. During this passage, a carboniferous residue is fonned and deposits on the tube walls and fittings.
The carbon deposits initially in a fibrous form. It is thought this results from a catalytic action primarily due to nickel and iron in the tube alloy. The fibrous carbon appears to form a mat on the tube wall. This traps pyrolytic coke particles that form in the gas stream. The result is build-up of a dense coke deposit on the tube wall. This carbon build-up is commonly referred to as "coking".
A short range concern is the thermal insulation of the tube wall. This necessitates continually increasing the fire box temperature to maintain a steady temperature in the hydrocarbon stream passing through the furnace. Ultimately, the fire box and tube wall reach temperatures where operation must be discontinued and the carbon removed in a procedure referred to as decoking.
A longer range concern is the effect of the carbon on the metal tubes in the cracking furnace. During operation, a furnace is under considerable pressure. The furnace tube may also be subjected to a considerable tensile load. This may arise, for example, due to a .DELTA.T across the tube wall during decoking, or during an automatic shutdown such as in an emergency.
During the life of a furnace, gradual embrittlement of the tube metal is observed with consequent loss of mechanical strength. Normally, pressure and tensile load remain relatively constant and not of concern. However, as a metal tube becomes weak due to embrittlement, these factors become significant. It then becomes necessary to shut the operation down and completely rebuild the furnace with new tubing.
Numerous solutions to the problems of coking and carburization have been proposed. One such solution involves producing the tubing from metal alloys having special compositions. Another proposed solution involves coating the interior wall of the tubing with a silicon containing coating such as silica, silicon carbide, or silicon nitride. Despite these numerous proposals, the problems still remain.
It would obviously be highly beneficial to be able to at least slow down the embrittlement process, and thus extend the life of the cracking furnace. It is a purpose of this invention to accomplish that desirable end. In a broader sense, a purpose is to provide a method of protecting metal alloys against embrittlement by contact with carbon or sulfur at elevated temperatures.